Mikuni Carb - 86 Sportster

[malc0lm]

Who know's their shit about Mikuni carbs in Auckland?

[BIG DOUG]

not in auckland,but whats the problem

[malc0lm]

It has a slow return to idle, takes maybe 15 to 30 seconds to return to idle, sometimes longer and sometimes quicker.

Also seems to backfire through the carb every now and then.

I've done the obvious stuff like checking that the throttle cable is not hung up all along its routing and that the throttle shaft nuts on the carb body are not too tight.

Also I though it might be an air leak, so I've replaced the seals between the intake manifold and head, also the flange between the carb and intake. Any idea's on how to further eliminate the problem as an air leak?

It's has a Mikuni carb stamped with – TO 3 1 - in one spot and just below stamped T40 6P.

Also checked the ignition timing as per the Clymer manual that I have. Do you think it could be a timing problem? I've tried my best to get the timing mark into the centre of the inspection hole. It never seems to be consistent though, when the strobe light flashes the timing mark is never in exactly the same spot, jumps around a little bit. Is that normal?

[nzspokes]

If its a new fault then clean your jets out. Sounds like a blocked idle jet. If not new then go up a size on idle jet.

[Paul in NZ]

Is it a slide carb or a CV carb?

[malc0lm]

I wouldn't say it's a new problem, been building a bobber for the last year and a half. Didn't run when I picked it up as a project bike.

I've cleaned it out and check it over, but I don't know much about carbs.

Put a lot of time and effort into this bike, that's why I want to take it to someone that knows their shit, also I'll know that it's in good hands.

[buggerit]

sticky slide? missing return spring?
Pop the aircleaner and have a watch with it running.

[husaberg]

It has a slow return to idle, takes maybe 15 to 30 seconds to return to idle, sometimes longer and sometimes quicker.

Also seems to backfire through the carb every now and then.

I've done the obvious stuff like checking that the throttle cable is not hung up all along its routing and that the throttle shaft nuts on the carb body are not too tight.

Also I though it might be an air leak, so I've replaced the seals between the intake manifold and head, also the flange between the carb and intake. Any idea's on how to further eliminate the problem as an air leak?

It's has a Mikuni carb stamped with – TO 3 1 - in one spot and just below stamped T40 6P.

Also checked the ignition timing as per the Clymer manual that I have. Do you think it could be a timing problem? I've tried my best to get the timing mark into the centre of the inspection hole. It never seems to be consistent though, when the strobe light flashes the timing mark is never in exactly the same spot, jumps around a little bit. Is that normal?

A slow return to idle" ie hanging" assuming it is not mechanical fault Ie(cable routing etc return spring etc air leak etc) suggests the circuit involved is lean. Carb diagnosis is pretty fraught over the internet though;)
that said I have no idea what carb it has on it.

but I googled this for you
The pilot circuit has two adjustable parts, fig 2. The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.
304444
Lastly the clymer manual will have the process

[pete-blen]

[QUOTE=malc0lm;1130783517]


Any idea's on how to further eliminate the problem as an air leak?

/QUOTE]


Engine start.....
spray it around the carb/intake maifold... if theres a air leak it
will be sucked in... the motor will pick up a few revs for a couple seconds..

[malc0lm]

Took the carb off last night, double checked all of the jets, blow everything out with the compressor, check the float bowl level, all is good there.

It's definitely not anything mechanical like cables holding it up.

If it still has this problem then sounds like it must be running lean ... will adjust it as per above and see if that helps as well.